This invention relates generally to systems for conditional access to television programming.
In a conditional access television system, access to the television programming is limited to certain viewers who pay to receive the programming. In some cases, the audio and video portions of the broadcast are digitally encrypted at the broadcast head end and decrypted at the receiver end.
In other cases, analog broadcast systems may be utilized. In analog systems, a graphics overlay may be provided at the head end over the analog television programming to make it difficult for one to watch the programming. Conventionally, the overlay provides distortions and horizontal lines that disrupt the picture so that very little content can be discerned.
Analog conditional access television systems generally do not distort the audio. In some cases, the audio may have considerable value. Thus, it would be desirable to also distort the accompanying audio information so that only authorized receivers may enjoy the audio.
Human perception of sound is much more precise than human visual perception. Artifacts at an acceptable level for the video portion of an analog transmission may be far too large for the audio portion. Moreover, analog television provides an audio channel as a frequency modulated (FM) subcarrier with only 0.75 MHz bandwidth.
There are numerous sources of noise and distortion in television broadcasts. These imperfections may disrupt the clean decoding of the audio signal. The most serious artifact is ghosting or multi-path transmission. Ghosting results at the receiver when the primary, first arriving signal is then followed by several delayed, weaker signals. These weaker signals may have been reflected off of intervening obstructions such as buildings, mountains or other structures. The primary, first arriving signal and the weaker signals sum together at the radio frequency input to the television decoder.
Each time a ghost signal is added to the television signal mix, the implicit value of an encoded signal bit is changed. Thus, naïve decoding of the audio signal produces large artifacts due to the ghost signals.
Even in cable distribution systems, improper signal splitters, bad cable terminations and other defects produce echoes that have the same effect as multi-path transmission over the air. As a result, these types of distortions may be prevalent in both wireless and wired transmissions.
Thus, there is a need for better ways to provide an analog conditional access television system that adequately protects the audio portion of the television transmission.